Good question Elizabeth, many people often ask themselves the same thing.
I would say that when teaching kids English it is good to be able to speak Spanish since it can get frustrating if they do not understand nor speak a lick of English yet and you may have to revert to explaining things in Spanish due to their short attention spans.
However, ideally a teacher of a second language should do everything possible to avoid using the native language of the learner (in this case Spanish) for it is detrimental to the learning of the second language.
Ok, for example, if you are teaching new vocabulary, it is far better to say present a new word like "car" and use other similar words in English (ie. automobile, vehicle) to make them understand what a car is rather than just saying "car is coche". Giving them the translation in their native tongue does not stimulate their minds to try and figure out the meaning on their own.
Acting out the word or using cue cards with pictures of the vocab words are also a preferential option to simply giving the translation. For example, if you teach them the verb "to run", get in front of them and run in place or show them a pic of a person ruinning instead of just saying "to run is correr".
So, while it is hard to not give them the answers in their native tongue, it is best not to. In theory the less Spanish you speak the better you will be able to teach them English (although you should have a minimal level of Spanish proficiency for emergencies).
The ideal language teacher does not necesarily have to speak the native tongue of his/her students, in fact, it is often better if he/she does not.
Hope that cleared things up a bit, just my opinion.
Saludos,
jer...
(near the end of his vacation and missing Madrid dearly

)